Energy storage Study Guide

📖 Core Concepts Energy storage – captures energy when it’s abundant and releases it later to match demand. Accumulator / battery – generic term for any device that stores energy. Storage forms – mechanical (gravitational, compressed air, flywheel), electrical (capacitors, SMES), chemical (batteries, fuels), thermal (sensible, latent, cryogenic). Round‑trip efficiency (η) – fraction of input energy recovered on discharge: $$\eta = \frac{E{\text{out}}}{E{\text{in}}}\times100\%$$ Energy‑on‑Energy Invested (ESOI) – energy stored ÷ energy required to build the device. --- 📌 Must Remember Pumped‑storage hydro provides >99 % of bulk storage worldwide; round‑trip 70–80 % (up to 87 %). Flywheel specific energy: 100–130 Wh · kg⁻¹ (≈ 360–500 kJ · kg⁻¹). Lithium‑ion ESOI ≈ 10; Lead‑acid ESOI ≈ 2; Pumped‑hydro ESOI ≈ 210. Supercapacitor energy density ≈ 10 % of batteries; power density 10–100× higher. SMES efficiency > 95 % (2–3 % inverter loss). Molten‑salt TES stores heat from concentrated solar; hot salt drives steam turbines. Ice‑storage chillers can be sized at 40–50 % of a no‑storage system’s capacity. --- 🔄 Key Processes Pumped‑storage cycle Low demand → electricity powers pump → water ↑ reservoir. High demand → water ↓ through turbine → electricity generated. Compressed‑Air Energy Storage (CAES) Surplus electricity compresses air → store in cavern. During discharge, air expands, driving a turbine; heat‑management mode (adiabatic/diabatic/isothermal) determines efficiency. Flywheel charge/discharge Motor accelerates rotor → kinetic energy stored as ½ I ω². Generator decelerates rotor → electrical output. Battery charge – electrochemical reaction proceeds in reverse of discharge; cell voltage given by Nernst equation (≈ 1–2 V per cell for most chemistries). Power‑to‑Gas (hydrogen) Electrolysis: 2 H₂O → 2 H₂ + O₂ (electricity → chemical). Methanation: H₂ + CO₂ → CH₄ + H₂O (adds 8 % loss). --- 🔍 Key Comparisons Pumped‑hydro vs. CAES – Pumped‑hydro: higher efficiency (70–80 %), proven tech, needs large elevation & water. CAES: lower efficiency (depends on heat recovery), can be built underground, works with existing caverns. Lithium‑ion vs. Lead‑acid – Li‑ion: high energy density, low self‑discharge, higher ESOI (≈10). Lead‑acid: cheap, low energy density, short lifespan under rapid discharge, ESOI ≈2. Supercapacitor vs. Battery – Supercapacitor: 10 % energy density, 10–100× power density, seconds‑to‑minutes discharge. Battery: higher energy density, minutes‑to‑hours discharge. Sensible‑heat vs. Latent‑heat TES – Sensible: stores energy via temperature rise; large mass needed. Latent: stores energy via phase change; high energy per mass, minimal temperature swing. --- ⚠️ Common Misunderstandings “All batteries have the same lifetime.” – Lifespan varies widely; lead‑acid degrades quickly under fast discharge, Li‑ion lasts thousands of cycles. “SMES can store unlimited energy.” – Energy limited by coil size and magnetic field strength; cost and cryogenic requirements restrict scale. “Hydro reservoirs only generate electricity.” – They also shift generation timing; efficiency stays high because water’s potential energy is unchanged. “Compressed air is always adiabatic.” – Real CAES may be diabatic (heat lost) or adiabatic (heat stored) – efficiency hinges on heat‑management. --- 🧠 Mental Models / Intuition Gravitational potential = mass × g × height → Think of pumped‑hydro and solid‑mass storage as “lifting a weight” and later letting it fall. Kinetic ↔ Rotational → Flywheel energy = ½ I ω²; faster spin = more energy, like a figure‑skater pulling in arms. Phase‑change plateau → Latent‑heat storage is a flat “temperature plateau” where heat goes into changing state, not raising temperature. Charge‑discharge symmetry – Batteries behave like reversible chemical “springs”; the farther you compress (depth of discharge), the more you get back, but wear increases. --- 🚩 Exceptions & Edge Cases Pumped‑hydro efficiency up to 87 % – only for optimized sites with minimal hydraulic losses. Flywheel specific energy 100–130 Wh · kg⁻¹ – achievable with carbon‑fiber rotors and vacuum; older steel rotors are far lower. SMES round‑trip >95 % – assumes superconducting state maintained; quench (loss of superconductivity) destroys stored energy. CAES modes – adiabatic (store heat for later) can push efficiency >70 %; diabatic (reject heat) drops to 50 %. --- 📍 When to Use Which Short, high‑power bursts (seconds–minutes): Supercapacitors or flywheels. Medium‑duration (minutes–hours) with moderate power: Lithium‑ion or flow batteries. Large‑scale, long‑duration (hours–days): Pumped‑hydro, CAES, molten‑salt TES, underground pumped‑hydro. Seasonal heating/cooling: Sensible‑heat storage in aquifers or boreholes; latent‑heat PCM walls. Transport fuels: Power‑to‑gas (hydrogen) for fuel cells; Power‑to‑liquid (methanol, ammonia) for aviation. --- 👀 Patterns to Recognize “Duck curve” → storage needed after sunset – look for solar‑dominant scenarios with evening peaks. High η + large capacity → mechanical bulk storage (hydro, pumped‑hydro). Low energy density + high power density → supercapacitors or SMES. Phase‑change material + flat temperature curve → latent‑heat TES. Electrolyte volume ↔ energy capacity → flow batteries scale by tank size. --- 🗂️ Exam Traps Choosing “SMES” for seasonal storage – SMES is great for seconds‑scale grid support, not months‑long storage. Assuming all batteries have >90 % round‑trip efficiency – many chemistries (e.g., lead‑acid) are 70–80 % and drop further at high discharge rates. Confusing “energy density” with “power density.” – Batteries excel at energy; supercapacitors excel at power. Treating CAES as 100 % efficient – ignore heat‑loss modes; efficiency depends on whether the system is adiabatic. Believing “hydro dams store electricity.” – they store potential energy; the electricity is generated only when water is released. ---